In general, a photosensitive plate for lithographic printing consists of a photosensitive layer comprising a photosensitive compound and a solvent-soluble binder resin as main component which is coated onto a substrate such as an aluminum plate. Such printing plate is then exposed through a photoimage (e.g. negative, etc.), and the nonexposed area of the photosensitive layer is removed by eluting with a developer and the exposed insolubilized area remains as the image area of the lithographic printing plate.
Photosensitive plates for lithographic printing have been widely used in offset printing, because the time taken for the plate-making process is short, and the operation is relatively simple. Accordingly, it is possible to provide a printing plate having excellent resolution, storage stability, developability and durability, and the demand for such a cheap photosensitive plate or resin composition used in lithographic printing is extremely large.
The following properties are required for a photosensitive resin having excellent properties which is used for making a photosensitive plate or photosensitive resin composition for lithographic printing:
(1) high sensitivity, that is, high absorptivity to active energy such that an image can be formed even when the exposure amount is small;
(2) stability, that is, the reactions (e.g. decomposition, crosslinking, etc.) will remain unchanged (even when it is stored for a long period of time at room temperature or under the conditions of high temperature and high humidity); and
(3) solvent-solubility, that is, it has excellent solubility to a developer containing no harmful solvent (e.g. DMF, methyl cellosolve, etc.).
Diazo resin has widely been used as a photosensitive compound for lithographic printing plate because it satisfies the above properties, comparatively. Diazo resin is a resin containing a diazonium salt. At present, a diazo resin is obtained by polycondensation of p-diazodiphenyl amine and paraformaldehyde with an inorganic or organic counter anion and is mainly used as photosensitive resin for lithographic photosensitive plate.
In order to further improve printing properties, particularly sensitivity, stability and solvent-solubility of the diazo resin, various trials have been made heretofore.
As a trial for high sensitization of the diazo resin, for example, a diazonium compound of which the main chain is polyester is described in Japanese Patent Kokai No. 54-30121. However, it is necessary to use a toxic compound (e.g. trifluoroacetic acid, etc.) as solvent in order to prepare a diazo resin from a diazonium compound, and a reaction process at high temperature for a long period of time is also necessary. Accordingly, such a method is not suitable for practical use.
In Japanese Patent Kokai No. 58-062641, a method comprising synthesizing a prepolymer from acetylamino aniline and then diazotizing the prepolymer to prepare a diazo resin is described. Further, in Japanese Patent Kokai No. 58-187925, synthesis of a diazo resin according to the reaction of a diazonium compound with a vinyl ester compound is described. However, these methods are complicated because there are many synthesis processes, which are not suitable for practical use. Further, such diazo resin is inferior in solvent-solubility and, therefore, the resulting photosensitive plate is inferior in developability.
In Japanese Patent Kokai No. 61-273538, there is described a diazo resin obtained by esterifying a diazonium compound having a hydroxyl group to bond with a carboxylic acid-containing compound, followed by esterification with styrene-anhydrous maleic acid copolymer at room temperature for 3 hours. However, since sensitivity is not sufficiently improved, the resulting photosensitive plate is inferior in resolution.
These conventional diazo resins normally exhibit high sensitivity to a light having a wavelength of about 220 to 400 nm, but their sensitivity to visible light having wavelengths of not less than 400 nm is insufficient. Accordingly, when the visible light is used for exposure, only part of the irradiation light is absorbed and, therefore, insufficient sensitivity can be obtained.
In order to improve the stability of the diazo resin, a trial for improving the water-insolubility and lipophilic nature of the diazo resin has been made by coupling a water-soluble diazo resin with an inorganic compound. In Japanese Patent Kokai No. 47-1167, coupling of a water-soluble diazo resin with an acidic aromatic compound or coupling of a water-soluble diazo resin with a phenolic aromatic compound containing a hydroxyl group is described. However, since resins obtained by these methods are inferior in solubility to glycol, ether, alcohol and ketone, developability of a photosensitive plate becomes inferior. Further, a sulfonic acid or sulfonate containing a long chain alkyl group used therein is deposited in the gum-like form on synthesis and, therefore, it is hardly produced.
Further, a trial for using a non-aqueous and organic solvent-soluble counter anion has also been made. In Japanese Patent Kokai No. 54-98613, halogenated Lewis acid (e.g. BF.sub.4, PF.sub.6, etc.) is used. In Japanese Patent Kokai No. 56-121031, perhalogen acid, perchloric acid and periodic acid are used. However, resins obtained by these methods are inferior in solvent-solubility. Accordingly, the developability of such photosensitive plate becomes inferior and the diazo resin remains on the non-image area even after development and, therefore, scumming arises during printing.
In Japanese Patent Kokai No. 59-38746, a trial for using an organic coupling agent in combination with an inorganic coupling agent is described. However, a resin obtained by such method is remarkably inferior in solubility to an alkaline developer. Accordingly, developability of a photosensitive plate becomes inferior and the diazo resin remains on the non-image area even after development and, therefore, scumming arises during printing.
In Japanese Patent Kokai No. 62-175731, a combination of two sorts of an organic sulfonic acid having not less than 2.5 basic units is described. In Japanese Patent Kokai No. 60-97350, the use of sulfopolyester as counter anion is described. According to these methods, stability of the resulting diazo resins is improved, however, reactivity of the diazo resin is lowered, which results in substantially insufficient resolution.
As a conventional trial for improving solvent-solubility of the diazo resin, for example, a co-condensation of an aromatic compound containing a phenolic hydroxyl group with an aromatic diazonium compound is described in Japanese Patent Kokai No. 1-102456. In Japanese Patent Kokai No. 1-102457, a co-condensation of an aromatic compound containing at least one sort of a carboxyl group and a hydroxyl group with an aromatic diazonium compound is described. Further, in Japanese Patent Kokai No. 1-245246, a co-condensation of an aromatic compound containing a sulfonic group, sulfinic group, and phenylene and naphthylene containing a salt thereof with an aromatic diazonium is described. However, when these resin compositions are used, the durability of the resulting printing plate becomes inferior.